The present invention relates to gas sensors, particularly to the leads connecting to the sensor.
Exhaust sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, exhaust sensors have been used for many years in automotive vehicles to sense the presence of exhaust gases. In automotive applications, the direct relationship between various exhaust gas concentrations and the air-to-fuel (A/F) ratios of the fuel mixture supplied to the engine allows the sensor to provide concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions. The management of exhaust emissions has become increasingly important because of the increased use of automobile engines.
One method of sensing exhaust gas uses electrochemistry. With an electrochemical method, there are two basic principles involved in gas sensing: the Nernst principle and the polarographic principle. Typically, an exhaust gas sensor utilizing an electrochemical method comprises an electrochemical pump cell and an electrochemical motive force cell.
With the Nernst principle, chemical energy is converted into electromotive force (xe2x80x9cemfxe2x80x9d). A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode with a porous protective overcoat exposed to exhaust gases (xe2x80x9csensing electrodexe2x80x9d), and a porous electrode exposed to a known gas""s partial pressure (xe2x80x9creference gas electrodexe2x80x9d). Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine""s exhaust. Also, a typical sensor has a ceramic heater attached to help maintain the sensor""s ionic conductivity. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:       E    =                  (                                                            -                RT                                                                                        4                ⁢                F                                                    )            ⁢              ln        ⁡                  (                                                                      P                                      O                    2                                    ref                                                                                                      P                                      O                    2                                                                                )                                where      :              
            ⁢      E        =          electromotive      ⁢              xe2x80x83            ⁢      force            R    =          universal      ⁢              xe2x80x83            ⁢      gas      ⁢              xe2x80x83            ⁢      constant                  F      =              Faraday        ⁢                              xe2x80x83                    ⁢                      xe2x80x83                          ⁢        constant              ⁢          
        ⁢    T    =          absolute      ⁢              xe2x80x83            ⁢      temperature      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢                        xe2x80x83                ⁢                  xe2x80x83                    ⁢      gas                  P              O        2            ref        =                                        oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                                          xe2x80x83                            ⁢                              xe2x80x83                                      ⁢            pressure                                                            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                                          xe2x80x83                            ⁢                              xe2x80x83                                      ⁢            reference            ⁢                                          xe2x80x83                            ⁢                              xe2x80x83                                      ⁢            gas                                          P              O        2              =                                        oxygen            ⁢                          xe2x80x83                        ⁢            partial            ⁢                          xe2x80x83                        ⁢            pressure                                                            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            exhaust            ⁢                                          xe2x80x83                            ⁢                              xe2x80x83                                      ⁢            gas                              
With the polarographic principle, the sensors utilize electrolysis whereby ions are sensed through a diffusion limiting current for aqueous electrolyte systems. The same approach can be applied to solid electrolyte systems for sensing gas species and for sensing of wide range air-to-fuel ratio of combustion exhaust gas systems. Generally, a sensor employing the polarographic principle is composed of a pair of current pumping electrodes where both are in contact with an oxide conductive solid electrolyte and one electrode is in contact with a gas diffusion limiting medium. The gas diffusion limiting means in conjunction with the pump electrodes create a limiting current which is linearly proportional to the measured gas concentration in the sample.
By combining the cell using a polarographic method (xe2x80x9cpump cellxe2x80x9d) and the cell using emf into one sensor, the sensor can be manufactured economically. However, a sensor of this type has a limited range of the air-to-fuel ratios covered as it is limited by the IR drop polarization of the electrolyte and the electrode potentials. The IR polarization is associated with the voltage gradient that is necessary, to drive the charged ions through the cell electrolyte, and the electrode potential to drive ions and electrons through the electrode and other conductive material. Typically, complex electronic circuits have been used to overcome the difficulties caused by the IR drop polarization of the electrolyte and the electrode potentials.
A known type of exhaust sensor includes a flat plate sensor formed of various layers of ceramic and electrolyte materials laminated and sintered together with electrical circuit and sensor traces placed between the layers in a known manner. Within the sensor, a flat plate sensing element is employed. This sensing element can be both difficult and expensive to package within the body of the exhaust sensor since it generally has one dimension that is very thin and is usually made of brittle materials. Consequently, great care and time consuming effort must be taken to prevent the flat plate sensing element from being damaged by exhaust, heat, impact, vibration, the environment, etc. This is particularly problematic since most materials conventionally used as sensing element supports, glass and ceramics for example, cannot withstand much bending. After the sensor is formed, exhaust gas can be sensed. With the use of ceramic materials, thermal shock resistance is a primary concern. This has an effect of influencing sensor manufacture because of the precautions taken to preserve during the sensor""s lifetime the fragile ceramic materials, i.e., to prevent cracking from thermal shocks, and the sensor""s electronics for heating control and sensing.
To prevent poisoning of exhaust sensor electrodes, exhaust gas poison resistance is maintained to protect the sensor during its lifetime. Sensor electrodes can be poisoned in various ways. For example, engine exhaust contain compounds such as silica, lead and other compounds that can poison the sensor. When the engine is in combustion mode, the sensor contacts exhaust. Particles from the engine parts or from other sources such as inferior fuel containing silica, lead and other compounds pass through the pores of the sensor""s protective layer and adhere to the surface of the sensor""s ceramics or adsorb on the surface of the electrode. This process can lead to the poisoning of the sensor causing deterioration of the sensor output and its response properties. To prevent poisoning, sensors have each electrode with poison protection. Sensor construction is more complex and difficult if each electrode maintains its own poison resistance. Accordingly, there remains a need in the art for a sensor having electrodes that share the same poison protection to render sensor fabrication simpler and more economic.
The deficiencies of the above-discussed prior art are overcome or alleviated by the gas sensor and method of producing the same.
A gas sensor, comprising an oxygen pump cell with a first pump electrode and a second pump electrode disposed on opposite sides of a first solid electrolyte layer and a second pump electrode. The sensor also comprises an emf cell with an emf electrode and a reference gas electrode disposed on opposite sides of a second solid electrolyte layer. The emf electrode is disposed in fluid communication to the second pump electrode. A via hole is disposed through the first solid electrolyte layer, such that the first pump electrode is in fluid communication with the second pump electrode. A protective insulating layer, having a passage for gas to be sensed, is disposed in contact with the first pump electrode. A first insulating layer, having a conduit, is disposed in contact with the emf electrode. A second insulating layer, having an air channel, is disposed in contact with the reference gas electrode. A heater is disposed in thermal communication with the emf cell. At least four electrical leads are in electrical communication with the sensor.
A method of producing a gas sensor, comprising providing an oxygen pump cell having a first pump electrode and a second pump electrode disposed on opposite sides of a first solid electrolyte layer. Also comprising disposing an emf cell having an emf electrode and a reference gas electrode disposed on opposite sides of a second solid electrolyte layer, wherein said emf electrode is in fluid communication to said second pump electrode. Disposing a protective insulating layer, having a passage for gas to be sensed, in contact with the first pump electrode. Positioning a first insulating layer, having a conduit, in contact with the emf electrode. Disposing a via hole through the first electrolyte layer such that the first pump electrode is in fluid communication with the second pump electrode. Disposing a second insulating layer, having an air channel, in contact with the emf cell. Positioning a heater in contact with the emf cell. Providing at least four electrical leads in electrical communication with the sensor.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.